This invention relates to blowing processes for refining molten metal in a vessel. Particularly, the invention relates to top blowing processes for improving removal of carbon, such as in a basic oxygen process.
It is known to produce ferrous metals in molten metal vessels wherein top blowing with oxygen through a lance positioned above the bath is used. For this purpose the vessel, such as a basic oxygen furnace, is typically charged with 60 to 80% hot metal, for example, from a blast furnace and 20 to 40% of a cold charge which may be high-carbon chromium alloy and/or stainless steel scrap. Top oxygen blowing is performed until the final bath carbon level has been reduced to approximately 0.035 to 0.05%; at which time the bath temperature is typically 3400.degree. to 3600.degree. F. (1871.degree. to 1982.degree. C.). At such carbon content, which may be currently achieved by the use of a top-blown basic oxygen converter, the bath temperatures are sufficiently high that excessive refractory wear occurs and thus charging of scrap for cooling of the bath is necessary. Presently, many product specifications require carbon levels less than 0.03%. The standard basic oxygen furnace practice cannot attain such low carbon levels.
It is also known, in top-blown oxygen steelmaking processes of this type, to blend an inert gas, such as argon, with the oxygen introduced by top blowing near the end of the blowing cycle. Although the argon serves to improve the efficiency of the carbon removal, nevertheless stainless steels having carbon contents less than about 0.03% may not be commercially produced on a consistent basis.
It has also been proposed to adapt a basic oxygen converter vessel for introduction of an inert gas to the bath from beneath the surface thereof by the use of tuyeres or porous plugs arranged on or near the bottom of the vessel. One practice would involve increasing the rate of inert gas introduced from beneath the surface of the bath and decreasing the oxygen introduced by top blowing of oxygen only as the refining operation progresses in the manufacture of steels. Such a method is disclosed in concurrently filed copending application Ser. No. 604,097, filed Apr. 26, 1984. Specifically, with stainless steel manufacture wherein an inert gas introduced beneath the bath surface is employed in combination with top-blown oxygen, the ratio of oxygen-to-inert gas is relatively high during initial blowing and must be decreased as blowing progresses. Initially, the rate of oxygen introduced is significantly higher than the rate of inert gas introduced; however, at the end of the blowing the rate of inert gas introduced is significantly higher than the rate of oxygen introduced. Therefore, the tuyeres positioned in the vessel for inert gas introduction must be capable of relatively high gas flow rates.
There have been proposals by others to use top-blowing processes only including oxygen and inert gas mixtures. U.S. Pat. No. 4,397,685, issued Aug. 9, 1983, describes a top-blowing process only which includes an oxygen-inert gas mixture, adjusting the flow mixture, and lowering lance height to achieve low carbon levels. U.S. Pat. No. 3,867,134, issued Feb. 18, 1975, discloses a process of top blowing oxygen, and then a mixture of oxygen and inert gas and varying the mixture composition. U.S. Pat. No. 3,307,937, issued Mar. 7, 1967, discloses top blowing only inert gas, then a mixture of oxygen and inert gas, and then finishing only inert gas. None of these patents, however, suggest the present invention.
An object of the invention is to provide a method of producing steel wherein the same top lances are used throughout the refining process although the overall oxygen-to-inert gas ratio of the process decreases progressively.
Another object is to provide a method whereby the relative gas flow between the top lances and the tuyeres or porous plugs remains relatively constant.
An object of the invention is to provide a method for producing steel wherein a relatively low inert gas flow rate is maintained through the tuyeres of the vessel.
These and other objects of the invention, as well as a more complete understanding thereof, may be obtained from the following description and specific examples.